In a telecommunications system such as a LTE system, the position of a user's wireless communication device (WCD) (e.g., a UE) may be tracked by the system's core network for purposes such as authorization of services, billing, customer care, or to comply with certain legal requirements. This location information, which approximates the location of a WCD, may be reported by a base station (e.g., an eNB) to a mobility management apparatus (e.g., mobility management entity (MME)) to which the WCD is attached. This location information allows the core network to track the WCD's location.
LTE makes available the following positioning strategies (i.e., position determination techniques): i) Enhanced Cell ID (e-CID); ii) Assisted Global Navigation Satellite System (GNSS); iii) Observed Time Difference of Arrival (OTDOA); and iv) Uplink Time Difference of Arrival (UTDOA).
In the e-CID approach, cell ID information associates a UE with a serving area of a serving cell. Additional information can improve granularity of the position determination.
In the Assisted GNSS approach, the UE may retrieve GNSS information, and may be supported by assistance information provided to the UE from an enhanced or evolved serving mobile location center (E-SMLC).
In the OTDOA approach, the UE may estimate a time difference of reference signals from different base stations, and send that time difference measurement to the E-SMLC for multilateration.
In the UTDOA approach, the UE is requested to transmit a specific waveform that is detected by multiple location measurement units (e.g., an eNB) at known positions. These measurements are forwarded to an E-SMLC for multilateration. The OTDOA approach is a WCD-assisted (e.g., UE-assisted) method, in which a WCD measures the time of arrival (TOA) of specific positioning reference signals (PRS) from multiple eNBs, and computes the relative differences. These reference signal time difference (RSTD) are quantized and reported via, for example, a LPP link to the E-SMLC together with an accuracy assessment. Based on known positions of eNBs and their mutual time synchronization, it is possible for the E-SMLC to estimate the WCD's position from the RSTD and covariance reports by using multilateration. The accuracy may depend on the radio conditions of the received signals, number of received signals as well as the deployment, which means that it may vary spatially.
The TOA estimation based on thresholding may give significant TOA estimation errors if the threshold is too high like in FIG. 4, or too low like in FIG. 5. It is difficult for one of ordinary skill in the art to specify a threshold-based algorithm for TOA estimation that can handle all possible cases without introducing a significant bias at times, which is significantly disadvantageous.
In existing solutions, the UE reported RSTD measurement and its corresponding accuracy assessment do not accurately reflect the error caused by multipath. Based on the PRS cross correlation, the UE associates an RSTD measurement with an uncertainty in meters, hence the format does not reflect the error characteristics from multipath and useful information is lost. As shown in FIG. 3, since the cross correlation typically gives a set of candidate time estimates (e.g., the distinct peaks), a reporting format that reflects the cross correlation results is therefore desirable.